NH$_3$ $(_g$) +  HCl$(_g$) →   NH$_4$Cl $_(g)$ In the reaction above, increase in pressure will

1. Analyze the question

The question asks how an increase in pressure affects the chemical equilibrium of the reaction: NH$_3(g) + \text{HCl}(g) \rightleftharpoons \text{NH}_4\text{Cl}(s)$ (Note: While the question writes NH$_4$Cl as a gas, in standard chemical equilibrium problems of this type, NH$_4$Cl is typically treated as a solid. Let's analyze the moles of gas on both sides.)

2. Explain the concept

According to Le Chatelier’s Principle, if a system at equilibrium is subjected to a change in pressure, the system will shift its equilibrium position to counteract that change.

• Increase in pressure: The system shifts toward the side with fewer moles of gas to reduce the pressure.
• Decrease in pressure: The system shifts toward the side with more moles of gas to increase the pressure.
• Equilibrium Constant ($K$): The equilibrium constant is only affected by temperature. Changes in pressure or concentration do not change the value of $K$.

3. Show the solution

• Step 1: Count the moles of gas on the reactant side. NH$_3(g)$ = 1 mole HCl$(g)$ = 1 mole Total = 2 moles of gas
• Step 2: Count the moles of gas on the product side. NH$_4$Cl$(s)$ = 0 moles of gas (solids are not counted in pressure equilibrium shifts).
• Step 3: Apply the pressure change. Since there are 2 moles of gas on the left and 0 moles of gas on the right, an increase in pressure will force the reaction to shift to the right (the product side) to reduce the total number of gas particles.
• Step 4: Evaluate the options. • Options A and D are incorrect because pressure does not change the equilibrium constant ($K$). • Option C is incorrect because increasing pressure favors the side with fewer moles of gas (the product). • Option B is correct because the reaction shifts toward the product side.

4. Identify the answer

The increase in pressure shifts the equilibrium to the right, thereby favoring the formation of the product.

5. Tip

Memory Trick: Think of pressure like a crowded room. If you increase the pressure (squeeze the room), the molecules will try to "shrink" by moving to the side of the equation that has fewer moles of gas to save space!